1. Field of the Invention
The present invention relates generally to the field of fluid dynamics and specifically to both the field of immersing low density and/or high surface area to volume solids into liquids and the field of moving fluids in a linear path.
2. Description of the Prior Art
Axial impellers are well known to those with skill in the field as a means for generally moving fluids in a direction which is parallel to the axis of rotation of such impellers. Axial flow impellers are generally categorized as one of two specific types: the first is a propeller, as conventionally used in marine applications; and the second is a turbine as conventionally found in various designs of liquid pumps. The marine propeller is generally characterized as being of a square pitch design, that is it has a variable angle and, therefore, an approximately constant radial pitch across the face of the impeller. The turbine, as distinguished, has a constant blade angle and therefore a variable radial pitch across the face of the impeller. Both types of impellers are used to move fluids in a generally linear direction.
It is well known that the operation of axial impellers, including both propellers and turbines, to varying extents, creates radial turbulence and ancillary radial flow, adjacent the circumferential periphery of the blades of the impeller, in a direction which is perpendicular to the impeller's axis of rotation. This radial turbulence tends to roll and tumble in a direction opposed to the direction of the linear flow of fluid passing through the impeller. The rolling and tumbling motion of the fluid created by the radial turbulence tends to roll and tumble into the path of the fluid entering the impeller, thus impeding and decreasing the linear flow of fluid into that impeller. The net result is that the speed of the impeller rotation must be increased to overcome the effects of the radial turbulence in order to maintain a desired volume of flow in a linear direction through the impeller. In addition, fluid which has just previously been passed through the impeller and radially expelled therefrom, followed by being rolled and tumbled in an opposite direction, tends to be immediately recirculated through the impeller, thus curtailing the flow of virgin fluid through that impeller. To move a desired volume of virgin fluid, per unit of time, through the impeller, the speed of the impeller's rotation must be even further increased. Thus, these increases in speed, combined with the radial turbulence and the rolling and tumbling motion of that turbulence, in an opposite direction, creates what is well known as a vortex effect.
A vortex effect is similar to the effect produced by a whirlpool and is characterized by much turbulence surrounding both the periphery of the axial impeller and the fluid entering that impeller. The vortex effect also tends to decrease the efficiency of the movement of fluid being expelled from the impeller in a linear direction, in that the rolling and tumbling action involved in the turbulence tends to redirect the linear flow into an arced or fanned direction.
The foregoing phenomena are good for localized mixing applications, using a stationary impeller, but are detrimental to systems where linear fluid movement is the object. In a marine application, using a propeller, the problems created by the turbulence of the vortex effect are overcome by the fact that the propeller moves along with its drive unit and the boat to which it is attached. Thus, the propeller is always moved forward ahead of the vortex effect and pushes against it. In a turbine application, such as a pump, the problem of the vortex effect is overcome by encasing the impeller into a stationary casing which closely surrounds the blades of the turbine and provides only an opening for the linear flow. Thus, if no radial flow can occur because of the closely adjacent encasement of the turbine, no vortex effect is created and the flow pattern is confined to a linear direction.
Axial flow impellers of both the propeller and the turbine design are commonly used in mixing apparatus, as inferred above, such as, for example, by placement of the impeller into a large tank with the walls of such tank being a substantial distance away from the blades of the impeller. If the impeller is placed near the surface of the fluid in such a tank, the vortex effect created by the radial turbulence can create a fluid void at the surface, in the form of a conical section converging from the surface of the liquid towards the center of the impeller. The flow of fluid surrounding the void creates a low pressure zone which causes the ambient atmosphere to be sucked into the impeller along with the fluid included in the vortex. Such an inclusion of ambient atmosphere can be detrimental in some applications. An example of such an application is often found where the specific problem is to entrain, into a fluid such as a liquid, either solids having a lighter density than the liquid, or solids having a relatively high surface area to weight ratio such that the surface tension of the liquid tends to hinder rapid sinking, by gravity, of such solids into the liquid. In such situations where it is important to exclude atmospheric gases from the liquid, but the solids "floating" on the surface of the fluid must be induced into the liquid, means are needed to accomplish that objective while eliminating the vortex effect.
If the purpose of the impeller is to linearly move fluid from one zone to another in a large tank, the vortex effect created thereby tends to hinder the efficiency of the inducement of such a linear flow. Thus, there are applications where there is a need for some means to reduce or eliminate the detrimental results of the vortex effect and to more efficiently move fluid in a linear direction.